Virtually all mammalian cells contain receptors for one or more classes of steroid hormones. These high-affinity receptor proteins most likely mediate the major effects of steroids on cell function by interacting with specific DNA sequences in (or near) target genes. Upon so doing, the steroid-receptor complex stimulates initiation of transcription thereby leading to increased production of specific RNAs. The study of complex regulatory mechanisms in higher eukaryotes has been hampered by the difficulty in manipulating such organisms genetically and by the enormity of mammalian genomes. We therefore have turned to simplified tissue culture systems and isolated genes to study the molecular mechanisms by which steroid hormones regulate gene expression. We will use genetic biochemical and molecular approaches to investigate the following issues: 1) What is the structure of the glucocorticoid receptor and which portions of the protein are required for its hormone binding, DNA binding, and transcription stimulating activities? 2) What are the biochemical components required for steroid-regulated transcription? 3) What factors determine whether a gene is or is not steroid responsive in a given cell (tissue) type? 4) How do steroid hormones alter the developmental pattern of gene expression in a well defined differentiating system? We will focus our attention on studies of the mouse glucocorticoid receptor and two specific genes that are regulated by glucocorticoids; one is the rat gene encoding the liver protein alpha-1 acid glycoprotein and the second is a mouse gene encoding a developmentally regulated cytochrome P450. It is our intent to eventually describe in detailed molecular terms the nature of the interactions between the glucocorticoid receptor and the transcriptional machinery of the cell. The approaches we are taking should help not only in elucidating the molecular mechanisms of steroid hormone action but may provide additional insights into the basis for tissue-specific gene expression and the process of cell differentiation.